1. Field of the Invention
The invention relates to fuel cells and in particular to polymer electrolyte membranes in fuel cells capable of operating at temperatures in excess of 120° C.
2. Description of the Prior Art
A fuel cell is an electrochemical device which reacts a fuel and an oxidant to produce electricity and water. A typical fuel supplied to a fuel cell is hydrogen, and a typical oxidant supplied to a fuel cell is oxygen (or ambient air). Other fuels or oxidants can be employed depending upon the operational conditions and type of fuel cell
The basic process in a fuel cell is highly efficient, and for those fuel cells fueled directly by hydrogen, pollution free. Further, since fuel cells can be assembled into stacks of various sizes, power systems have been developed to produce a wide range of electrical power outputs and thus can be employed in numerous industrial applications. The teachings of prior art patents, U.S. Pat. Nos. 5,242,764; 6,030,718; 6,096,449, are incorporated by reference herein.
A fuel cell produces an electromotive force by reacting fuel and oxygen at respective electrode interfaces which share a common electrolyte. For example, in proton exchange membrane (PEM) fuel cells, the construction of same includes a proton exchange membrane which acts not only as an electrolyte, but also as a barrier to prevent the hydrogen and oxygen from mixing. One commercially available proton exchange membrane is manufactured from a perfluorcarbon material which is marketed under the trademark Nafion, and which is sold by the E. I. DuPont de Nemours Company. Proton exchange membranes may also be purchased from other commercial sources. As should be understood, the proton exchange membrane is positioned between, and in contact with, the two electrodes which form the anode and cathode of the fuel cell.
In the case of a PEM type fuel cell, hydrogen gas is introduced at a first electrode (anode) where it reacts electrochemically in the presence of a catalyst to produce electrons and protons. The electrons are circulated from the first electrode to a second electrode (cathode) through an electrical circuit which couples these respective electrodes. Further, the protons pass through a membrane of solid, polymeric electrolyte (a proton exchange membrane or PEM) to the second electrode (cathode). Simultaneously, an oxidant, such as oxygen gas, (or air), is introduced to the second electrode where the oxidant reacts electrochemically in the presence of the catalyst and is combined with the electrons from the electrical circuit and the protons (having come across the proton exchange membrane) thus forming water. This reaction further completes the electrical circuit.
The following half cell reactions take place:H2→2H++2e-(½)O2+2H++2e-→H2O
As noted above the hydrogen or fuel-side” electrode is designated as the anode, and the oxygen (skip “side”) electrode is identified as the cathode. The external electric circuit conveys the generated electrical current and can thus extract electrical power from the cell. The overall PEM fuel cell reaction produces electrical energy which is the sum of the separate half cell reactions occurring in the fuel cell less its internal losses.
Polymer electrolyte membrane fuel cells are promising as power sources for transportation applications. In recent years, great strides have been made in the development of reformate-air fuel cells. Perfluorinated ionomeric membranes such as Nafion®) have been widely used in PEM fuel cells as electrolytes due to the excellent stability, high ionic conductivity and mechanical strength that these polymeric materials offer. This is particularly true for stack operation below 120° C.
More recently, there is a new emphasis on increasing the temperature of fuel cell operation to 150° C. or even as high as 200° C. so that carbon monoxide tolerance can be enhanced from the current levels of 100 ppm to 10,000 ppm. However, at temperatures greater than 120° C., the water retentivity of Nafion-type membranes is poor. Consequently, the ionic conductivity of Nafion suffers resulting in poor fuel cell performance.
Thus, an alternate membrane that retains high conductivity at temperatures as high as 200° C. is needed.
It is also important that such a membrane exhibits sufficient thermal and electrochemical stability and favorable interfacial properties for electro-reduction of oxygen and electro-oxidation of hydrogen.